The present invention relates to a crosslinkable ethylene copolymer composition for rotational molding. More particularly, the present invention is concerned with a crosslinkable ethylene copolymer composition for rotational molding, substantially free of air bubbles formed in rotational molding and capable of affording molded products superior in impact resistance.
Heretofore, the rotational molding method has widely been used for the production of hollow containers and other products of large size and complicated structure which cannot be molded by the blow molding method, such as, for example, gasoline tanks, casks, storage tanks and windsurfing boards.
The rotational molding method comprises the steps of feeding a synthetic resin powder to a mold, heating the mold from the exterior while rotating the mold uniaxially or biaxially to melt the powder, thereby forming a molten synthetic resin layer within the mold, then cooling the mold to solidify the synthetic resin layer and removing the mold from the thus-molded product.
The rotational molding method is simply based on the principle that a molten resin flows with rotation of a mold to form a molten resin layer on the mold surface. Therefore, it is necessary to use a resin having a good fluidity, or else it will be impossible to obtain a molded product having a good appearance, for example, the molded product obtained will contain air bubbles or have an uneven inner surface. Thus, it is important to use a resin having superior melt-flow characteristics capable of forming a uniform layer.
On the other hand, the resultant molded product is required to have superior physical properties such as high impact strength and resistance to environmental stress cracking.
As a method of satisfying both such superior fluidity and superior physical properties of product, it has heretofore been known to incorporate a cross-linking agent, e.g. a peroxide, into a polyethylene composition. According to this known method, polyethylene first becomes fluidized and provide a uniform coating over the mold inner surface and is then crosslinked by the peroxide, whereby the physical properties of the final product are improved.
In an effort to improve the impact strength and other physical properties, there has also been proposed a composition containing, in addition to polyethylene, rubber or elastomer, a crosslinking assistant such as 1,2-polybutadiene and a crosslinking agent (see Japanese Patent Laid Open Nos. 56143/1980, 135848/1982 and 147445/1983). Having made studies on this respect, the present inventors found that the addition of such crosslinking assistant as 1,2-polybutadiene could greatly increase the crosslinking efficiency and afford bubble-free rotational moldings, but caused a marked reduction of impact strength.
It has also been known that the impact strength can be improved by the addition of rubber or elastomer. However, the addition of such substance causes reduction of rigidity, and this reduction of rigidity is a serious problem particularly in large-sized moldings which are produced mainly by the rotational molding method.